They are actually quite different...
Varranger is meant to be stand alone and aims to play styles of all major parties..
Calisto is something you integrate with your DAW and use to create your own styles and performances... with only 6 style tracks it will at first not be compatible with other style brands not do i see an import function..
Calisto is something i would like to see in action with Reason 9.5... which due to its modularrity adds a whole pletora of options